Processes of leaching mineral bearing ores to recover valuable products are known. Typically, a lixiviant solution is introduced to the ore and the resulting liquid, i.e., leach liquor, is then processed to remove the desired products. Manganese bearing ores are particularly amenable to leaching as a method to remove the manganese contained in the ores.
Manganese, in form of manganese carbonate, is an important raw material component in the manufacture of regenerable sorbent pellets for the removal of hydrogen sulfide from hot coal-derived fuel gases. Manganese carbonate can also be easily integrated as a feed material to existing electrolytic manganese and electrolytic manganese dioxide plants.
A number of studies on the manganese-sulfur-oxygen (Mn--S--O) system have indicated that a Mn-based sorbent pellet has considerable potential application in the desulfurization of coal-derived fuel gases at high temperature. A MnCO.sub.3 based sorbent pellet offers a highly desirable combination of reaction kinetics and sulfur loading capacity.
When forming sorbent pellets for the desulfurization of gases, it is particularly important that the manganese carbonate be as pure as possible and that impurities, such as iron, be minimized to provide pellets which capture high amounts of sulfur and can be regenerated for reuse over a number of cycles. Under reducing conditions, the iron-oxygen-sulfur (Fe--S--O) system forms a series of low melting-point liquid solutions which inhibit sorbent pellet reactivity with sulfur. Iron removal is, therefore, important for maximizing the desulfurization potential of manganese-based sorbent pellets.
Although reductive acid leading is a known method of obtaining manganese from ores, one problem with acid leaching processes is, however, the leaching of impurities along with the manganese. If the ore contains iron, which it typically does, the leaching process also removes a significant amount of iron in addition to manganese, thereby complicating processing of the leach liquor to selectively remove the manganese without significant quantifies of iron.
Products containing manganese and iron may be useful if used as a steel additive, but that use does not economically justify the processing necessary to extract the manganese and iron together. For these reasons, treatment of acid leach solutions should be focused on producing manganese in the form of carbonate which is substantially free of base metal impurities such as iron.
Several procedures separating nickel, copper, cobalt and iron from deep sea nodules in chloride-based and sulfate-based systems have been developed. Many of those processes, however, are not focused on the recovery of manganese.
One process for extracting manganese from deep sea nodules using a hydrochloric acid-based leach liquor has been developed by Metallurgie Hoboken-Overpelt. After solubilization of the nodules in strong hydrochloric acid which produces manganous chloride, chlorine is used to oxidize the manganous chloride under conditions of controlled pH by additions of magnesia to precipitate manganese as a dioxide. The leach solutions were first purified of iron, zinc, copper, aluminum, nickel, and cobalt by a series of prior selective precipitation steps which include adding sulfuric acid followed by sulfide precipitation. This process depends upon the initial presence of manganese in the dioxide form to generate chlorine in the leaching reactors.
Another process for recovering manganese from leach liquors produced by sulfuric acid leaching of manganiferous iron ores is known. The starting solutions had a reduction potential of -310 mV vs. saturated calomel. The more rapid oxidation kinetics of iron over that of manganese were used by aerating the solution at a controlled pH to precipitate iron selectively and purify the manganese solution. At a pH of 7.25 and an aeration time of 15 min, a solution initially containing 28 g/l Mn and 3.8 g/l Fe was separated into a treated liquor containing 28 g/l Mn and less than 0.1 g/l Fe. After 45 minutes of aeration, some loss of manganese to the precipitated iron occurred.
In one series of tests, recovery of manganese was by electrowinning at the cathode; however, and in separate tests, recovery of manganese was by electro-oxidation and deposition of manganese dioxide at the anode of an electrolytic cell. In any event, the amounts of iron remaining in the treated liquors which contained substantially all of the initial manganese, i.e., 0.1 g/l of Fe, was too high for processing into manganese based sorbent pellets for the desulfurization of coal-derived fuel gases.
In another process manganese was extracted from low grade ores by calcining in the presence of ammonium sulfate followed by water-leaching to produce a solution of a relatively low iron content and a high manganese content by careful control of the calcination conditions. The resulting solutions were then oxidized by contacting them with fresh ore containing manganese dioxide to convert the iron to the ferric state. Iron was then selectively precipitated at a pH of 6.5 by ammonia addition. Finally, manganese was recovered at a pH of 8.5 by precipitation as manganese carbonate. After treating the manganese carbonate with nitric acid and baking at 200.degree. C. To remove the nitrates, a pure manganese dioxide product resulted.
A combination process for the recovery of both metallic iron and metallic manganese from manganiferous iron ores of the Cuyuna range has also been developed. In the process, iron is recovered by direct reduction and magnetic separation, and the manganese is recovered from the nonmagnetic tailing by leaching and electrolysis. The initial electrolytic solutions typically contained about 28 g/l manganese with the depleted solutions containing 17 to 18 g/l for only a 35% drop in solution concentration. The disadvantages of this process include current efficiencies of only 60 percent and the requirement for a significant recycle stream of lixiviant, both of which increase the cost of obtaining the manganese present in the ore.
Yet another process for recovering manganese involves a precipitation separation method to separate copper, nickel, cobalt and manganese from sulfurous acid leach liquors resulting from leaching of deep sea nodules. A slight excess of sulfurous acid is used in the leach process to reduce all the manganese to the divalent state. Iron is then oxidized under ambient conditions by aeration and addition of Na.sub.2 CO.sub.3. The iron is then precipitated as ferric hydroxide by adjustment of the pH of the leach liquor to values in the range of 4.0 to 4.9. Subsequently, manganese is precipitated as MnCO.sub.3 by the addition of ammonium carbonate which raises the pH value to 9-10. The copper, nickel, and cobalt remained in solution since they had been stabilized as amine-complexes. After several trials, over 90% recoveries of copper, nickel, and cobalt were obtained and about 97% of the manganese and 99.9% of the iron were removed.
One disadvantage of this process includes the use of ammonia which is expensive and difficult to recycle for further use, thereby eliminating the ability to reduce costs by reusing the ammonia and adding to the costs of disposal.